Applicant's invention relates to communication systems in which information in the form of digital words is transmitted, and more particularly to systems in which each word is transmitted several times.
Some current cellular mobile telephone systems use analog frequency modulation (FM) of a radio-frequency (RF) carrier signal to transmit speech and other information. One of the standards is the Advanced Mobile Phone Service (AMPS) system in the U.S. that uses FM and a spacing between 880-MHz carrier signals of 30 KHz. Some of the characteristics of the AMPS system are specified by the EIA/TIA-553 standard published by the Electronic Industries Association and Telecommunications Industry Association (EIA/TIA). Another standard is the Total Access Communications System (TACS) in the United Kingdom that uses spacings of 25 KHz between 880-MHz carrier signals. The TACS and AMPS system differ in a few details that are not relevant to this application.
The capacity of such analog FM systems can be increased by reducing the channel bandwidth, as in the system according to the Narrow-band AMPS (N-AMPS) standard. In an N-AMPS system, a channel spacing of 10 KHz is achieved by splitting each 30-KHz-wide radio channel of an AMPS system into three parts. In general, a radio channel is a bi-directional radio transmission path between two transceivers, and thus the channel comprises two carriers having different frequencies, one for uplink communication (mobile station to base station) and one for downlink communication (base station to mobile station). In the standard systems, the frequency spacing between the two carriers of any radio channel is 45 MHz.
In another effort to increase system capacity, a digital AMPS (D-AMPS) system in the U.S. uses digital transmission and time-division multiple access (TDMA) on traffic channels while keeping analog transmission on control channels. In a TDMA communication system, a radio channel is divided into a succession of time slots, each of which contains a burst of information from a data source, e.g., a digitally encoded portion of a voice conversation. Successive time slots assigned to the same user, which are usually not consecutive time slots on the radio carrier, constitute the user's digital traffic channel, which may be considered a logical channel assigned to the user. Some of the characteristics of the D-AMPS system are specified by the IS-54-B standard published by the EIA/TIA.
In systems according to these analog standards, control channels are used for setting up calls, informing base stations about locations and parameters associated with mobile stations, and informing mobile stations about locations and parameters associated with base stations. Voice or traffic channels are used for carrying voice conversations and sometimes for data.
FIGS. 1A, 1B illustrate an exemplary multi-layered cellular communication system that can use any of the above-described systems. An umbrella macrocell 10 represented by a hexagonal shape (see FIG. 1A) is part of an overlying cellular structure comprising many macrocells A.sub.1 -A.sub.7, B.sub.1 -B.sub.7 (see FIG. 1B). Each umbrella cell may contain an underlying microcell structure. The radio coverage of the umbrella cell and an underlying microcell may overlap or may be substantially non-overlapping. The umbrella cell 10 includes microcells 20 represented by the area enclosed within the dotted line and microcells 30 represented by the area enclosed within the dashed line corresponding to areas along city streets, and picocells 40, 50, 60, which cover individual floors of a building.
FIG. 2 is a block diagram of an exemplary cellular mobile radiotelephone communication system for use with the cellular structure shown in FIGS. 1A, 1B. The communication system includes a base station 110 that is associated with a respective one of the macrocell, microcell, or picocell; a mobile station 120; and a mobile services switching center (MSC) 140. Each base station has a control and processing unit 130, which communicates with the MSC 140, which in turn is connected to the public switched telephone network (not shown). Each base station also includes at least one voice channel transceiver 150 and a control channel transceiver 160, which are controlled by the control and processing unit 130. The mobile station 120 includes a similar voice and control channel transceiver 170 for exchanging information with the transceivers 150, 160, and a similar control and processing unit 180 for controlling the voice and control channel transceiver 170.
Complicating the task of any radio communication system is interference between the several radio transmitters and the variability of the radio channel caused by relative motion of the system users, changing radio signal reflections from structures and vehicles, resulting in fading, etc. Such interference and channel variability contributes to errors in the information communicated, and much effort is expended to overcome these errors. For example, some cellular radiotelephone systems convert analog information to be transmitted into digital information, which is then transformed, or encoded, in a way that permits some errors to be corrected.
As one specific example, linear Bose-Chaudhuri-Hocquenghem (BCH) codes can be decoded with low complexity using either the Peterson-Gorenstein-Zierler procedure or the Berlekamp-Massey procedure, or any procedure for decoding cyclic codes, as described in R. Blahut, Theory and Practice of Error Control Codes, chapt. 7, Addison-Wesley, Reading, Mass. (1983). Such a code is specified by code parameters (n, k) with minimum Hamming distance d and with code symbol alphabet GF(2.sup.m). In essence, such a code transforms a block of k data bits into a larger block of n coded bits. The additional bits are usually called parity bits, CRC bits, or check bits. The error correcting capability t of the code, i.e., the number of errors that the code can correct, is given by t.ltoreq.[(d-1)/2].
The TACS and AMPS and N-AMPS systems address the problem of interference and channel variability with such error correction coding and with redundancy. For example, a word, or block of data, to be transmitted is first encoded with a BCH code, and then the encoded word is transmitted five times. At the receiver, a majority vote is taken of each set of corresponding bits to determine the received word: corresponding bits of the five words received are compared, and for each bit position, the bit value having the majority of occurrences is taken as the bit value of the received word. This is called bit-wise majority voting in this application.
U.S. Pat. No. 5,568,513 filed May 11, 1993, entitled "Standby Power Saving in Mobile Phones" describes receiving five repetitions of a word and decoding each repetition as it is received. Upon successfully decoding one of the repetitions, the remaining repetitions are ignored; if none of the repetitions is successfully decoded, then a bit-wise majority vote of the five repetitions is taken and the voting result is decoded.
An example of bit-wise majority voting is illustrated by the following table, which shows five data words and their associated parity bits received, and the received word and parity bits resulting from a simple bit-wise majority vote.
______________________________________ Data Bits Received Parity Bits Received ______________________________________ 101010101010110 11110101 111010101000101 01010101 101011010100101 01010101 101001010101010 01010100 010101001010101 01010101 bit-wise majority vote: 101011001000101 01010101 ______________________________________
The bit values determined by the bit-wise majority vote are deemed to constitute the received word, which is passed to a decoder. As described below, the BCH code used by the TACS permits one-bit and two-bit errors to be corrected; if more than two bits of a word are in error, the decoder reports the word is incorrectable.
The shortcoming of such bit-wise majority voting of all of the words received is that if two of the five words received each contain more than two errors, the majority vote may produce a word that the decoder will report as incorrectable.